Upon applying the brake, locking of vehicle wheels is generally observed, which decreases the frictional force between the road surface and the tire to thereby diminish braking force and weaken directionality of the vehicle. Gravity value is known as a kind of anti-skidding device for preventing this phenomenon. This device decreases the tendency of rear-wheel locking caused by decreasing of load thereupon (nose diving), when the brake is applied, because of forward shifting of vehicle gravity due to inertia. The basic principle of the device resides in that a ball placed on a slope rolls up therealong by inertia, in case of brake applying, to shut off the path of braking-fluid to the rear brake. A few modifications of the device are known in U.S. Pat. Nos. 3,143,125 and 3,147,045 and others.
When a tandem cylinder is used as a master cylinder, in which a pair of pistons are fitted in series, for simultaneously producing two differently directed braking hydraulic forces, in case of a breakdown taking place in one of the two hydraulic circuits, hydraulic pressure in the other circuit will not rise enough before the braking-fluid in the damaged circuit is completely exhausted out therefrom. This causes the time for the entire stroke of the brake pedal to be longer than in normal condition, and in addition makes the stroke effective only at the final stage thereof, possibly incurring a great deal of danger.
The present invention is aimed at the provision of a safety device for use in a vehicle hydraulic brake having, with the object of eliminating such a danger, double functions as a gravity valve and as a differential valve with a shut-off valve, in which a differential piston is axially movably fitted within a valve casing, with the opposite end surfaces thereof receiving respectively the hydraulic pressure from each of the two circuits, to slide in the axial direction due to the pressure difference in the braking-fluid caused by the breakdown in any one of the hydraulic circuits for shutting off the path of the braking-fluid to the damaged circuit and alarming the driver of the breakdown or damage in the hydraulic brake system.